Re: [PATCH v2] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive

[Yongji Xie]

On 2016/5/11 23:10, Bjorn Helgaas wrote:

> On Wed, May 11, 2016 at 07:34:19PM +0800, Yongji Xie wrote:
> > Current vfio-pci implementation disallows to mmap
> > sub-page(size < PAGE_SIZE) MMIO BARs because these BARs' mmio
> > page may be shared with other BARs. This will cause some
> > performance issues when we passthrough a PCI device with
> > this kind of BARs. Guest will be not able to handle the mmio
> > accesses to the BARs which leads to mmio emulations in host.
> >
> > However, not all sub-page BARs will share page with other BARs.
> > We should allow to mmap the sub-page MMIO BARs which we can
> > make sure will not share page with other BARs.
> >
> > This patch adds support for this case. And we try to add some
> > dummy resources to reserve the remaind of the page which
> > hot-add device's BAR might be assigned into.
> This is starting to look more reasonable from a safety perspective.
> At least I don't have an allergic reaction to mapping a page that may
> contain BARs from other random devices :)
>
> > Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/vfio/pci/vfio_pci.c         |   85 ++++++++++++++++++++++++++++++++---
> >   drivers/vfio/pci/vfio_pci_private.h |    8 ++++
> >   2 files changed, 86 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> > index 98059df..33282b8 100644
> > --- a/drivers/vfio/pci/vfio_pci.c
> > +++ b/drivers/vfio/pci/vfio_pci.c
> > @@ -110,16 +110,77 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
> >   	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
> >   }
> >   
> > +static bool vfio_pci_bar_mmap_supported(struct vfio_pci_device *vdev, int index)
> > +{
> > +	struct resource *res = vdev->pdev->resource + index;
> > +	struct vfio_pci_dummy_resource *dummy_res1 = NULL;
> > +	struct vfio_pci_dummy_resource *dummy_res2 = NULL;
> > +
> > +	if (IS_ENABLED(CONFIG_VFIO_PCI_MMAP) && res->flags & IORESOURCE_MEM &&
> > +		resource_size(res) > 0) {
> > +		if (resource_size(res) >= PAGE_SIZE)
> > +			return true;
> > +
> > +		if ((res->start & ~PAGE_MASK)) {
> > +			/*
> > +			 * Add a dummy resource to reserve the portion
> > +			 * before res->start in exclusive page in case
> > +			 * that hot-add device's bar is assigned into it.
> > +			 */
> > +			dummy_res1 = kzalloc(sizeof(*dummy_res1), GFP_KERNEL);
> Should check for kzalloc() failure here.
>
> > +			dummy_res1->resource.start = res->start & PAGE_MASK;
> > +			dummy_res1->resource.end = res->start - 1;
> > +			dummy_res1->resource.flags = res->flags;
> > +			if (request_resource(res->parent,
> > +						&dummy_res1->resource)) {
> > +				kfree(dummy_res1);
> > +				return false;
> > +			}
> > +			dummy_res1->index = index;
> > +			list_add(&dummy_res1->res_next,
> > +					&vdev->dummy_resources_list);
> The main body of this function is unnecessarily indented.  If you did
> this it would be less indented:
>
>    if (!IS_ENABLED(CONFIG_VFIO_PCI_MMAP))
>      return false;
>
>    if (!res->flags & IORESOURCE_MEM)
>      return false;
>
>    /*
>     * Not sure this is necessary; the PCI core *shouldn't* set up a
>     * resource with a type but zero size.  But there may be bugs that
>     * cause us to do that.
>     */
>    if (!resource_size(res))
>      return false;
>
>    if (resource_size(res) >= PAGE_SIZE)
>      return true;
>
>    if ((res->start & ~PAGE_MASK)) {
>      ...

Thanks for your comments. I'll send a v3 soon.

Regards,
Yongji